1. Field of the Invention
The invention relates to machine tools for machining rotating workpieces.
In recent years there has emerged a need to machine workpieces to non-uniform shapes on a production basis, with the usual requirements of a high accuracy of finish and a maximum speed of production. Such a requirement exists, for example, in the field of production of pistons, piston rings and bearings for internal combustion engines. In the past, many pistons and piston rings produced on a commercial scale have been generally cylindrical while bearings have been either cylindrical or generally flat. It has recently been discovered, however, that the performance of pistons, piston rings or bearings can be improved substantially by shaping the working surface. For example, the performance of a piston and of an engine including such a piston can be improved by barrelling the piston so that it has a maximum diameter intermediate its ends and minimum diameters at its ends. Additionally or alternatively, this performance can also be improved by making the piston non-circular, for example oval or elliptical or by providing other non-uniform features. In general, the exact shape of a piston, piston ring or bearing required to maximise the performance of a particular engine to maximise the performance cannot be predetermined and it is necessary to test a number of alternative configurations in order to optimise performance. In addition, once an optimum configuration has been found, it is necessary to produce the item concerned in substantial quantities.
There has thus emerged a requirement for a machine tool which is capable of machining workpieces rapidly and accurately to have required non-uniform profiles. It will be appreciated that such a machine must have the capability of altering the tool position a number of times within a revolution, if it is to machine oval and elliptical profiles and must be able to do this at high speeds, for example, with the workpiece rotating at 1000-3000 rpm so necessitating accurate tool movements measured in microseconds. In effect, therefore, the tool must be able to be oscillated at high frequencies. In addition, the machine tool must be able to be adapted readily to machine differing profiles.
2. Review of the Prior Art
These requirements are not met by a conventional machine tool such as is shown in British Patent Specification No. 829,824 and which comprises a tool holder movable relatively to the rotating workpiece in directions parallel to and transverse to the axis of rotation of the workpiece by means of a servo system including respective lead screws extending between associated nuts fixed on a conventional tool slide and associated drives for the lead screws such as a motor or a manually operable handle. Such tool holding slides have a wide range of movement, for example a maximum movement of from 250 mm to 6500 mm and can apply a wide range of cutting forces up to a high maximum cutting force which may, for example, be about 1100 kg. They are thus capable of performing many machining tasks.
However, because of the size and inertia of the slide and the lag and backlash inherent in the lead screw and motor drive system, such an arrangement has a very narrow bandwidth and thus the tool cannot be moved relatively to the workpiece rapidly by small increments of distance. Thus, such a tool does not meet the requirements for machining and producing non-uniform workpieces at commercial rates.
As well as such conventional machine tools, there are more specialised machine tools of which three categories will now be discussed: grinding machines, that so called "Swiss lathes" and cam follower machines.
An example of a grinding machine is shown in Bristish Patent Specification No. 1,499,812 (Cranfield Institute of Technology). In this arrangement, the workpiece is shaped by a motor-driven grinding wheel which is mounted for movement by a lead screw driven by a motor. Although the servo system of this Specification has a maximum cutting force of 450 kg and a bandwidth which is wider than the bandwidth of a machine tool of the conventional type referred to above, the weight, and thus the inertia of the motor which drives the grinding wheel and the lag and backlash inherent in the continued use of a lead screws still prevents the oscillation of the tool necessary to produce non-uniform profiles at exceptionally high rates.
Examples of the so-called `Swiss lathe`, are shown in U.S. Pat. Specifications Nos. 3,680,415 and 3,688,612. In the Swiss lathe, a number of tools surround the workpiece and are moved into and out of engagement with the workpiece by respective cams carried on a common shaft and driven by a stepping motor through a gearbox. There is a clutch system provided which allows any tool to be drivingly engaged with the associated cam to move the tool into position in accordance with positioning commands.
As with a conventional machine tool, however, this system is designed to move the tool into a predetermined machining position and is not capable of moving the tool within a revolution of a workpiece rotating at high speed. This is reflected in the construction of the tool moving system where the presence of the gearbox, the number of cams on a common shaft and the clutch system make oscillating movements impossible.
An example of a cam follower machine is shown in British Patent Specification No. 1,461,321 (Regie National Des Usines Renault). In this system, there is provided a rotating cam which has a shape which is the shape of the desired final profile of the workpiece. The cam is contacted by a servo system formed by a cam follower whose motion is transmitted to the tool by a linkage system. Although such a tool can cut non-uniform workpieces at rotational speeds which are higher than the above-mentioned conventional machine tool, there is an upper limit to the rotational speed of the cam and the workpiece due to the fact that, at high speeds, the follower will leave the cam surface, thus causing errors in the profile of the workpiece. This tendency of the follower to leave the cam prevents certain non-uniform shapes being machined, for example, shapes which have rapid changes of profile. The maximum cutting force of such a system is low although the bandwidth is higher than a conventional machine. In addition, a fresh cam has to be prepared if a changed workpiece profile is required. This is both time consuming and costly since such cams are generally made using special precision equipment.
All these machine tool systems thus suffer from the disadvantage that they are unable to move the tool position at very high speed because of inherent lag and inertia in the servo system caused in general by the weight or size of the tool holder and the transmission system between the tool holder drive and the tool holder.
There exists, therefore, a need for a machine tool system which can machine workpieces to non-uniform profiles at very high speeds.